The present invention relates to an apparatus for monitoring frequency of WDM signals. In particular, the present invention relates to an apparatus monitoring frequency of WDM signals using pilot tone and two interleaved solid etalon filters for efficient management and maintenance of wavelength division multiplexing optical transmission network.
Optical transmission networks with wavelength division multiplexing employ a number of lasers of different wavelengths in order to increase transmission capacity of each optical fiber.
In Optical transmission networks with wavelength division multiplexing scheme, even though lasers operate on low transmission speed, transmission capacity per optical fiber can be dramatically increased. Therefore, conventional optical fibers that are already installed don""t need to be replaced.
However, since multiplexers and demultiplexers are sensitive to frequency of each channel and output level of each channel is heavily related with overall performance of system in wavelength division multiplexing optical transmission network, the frequency monitoring equipment is necessary for efficient management of optical transmission network.
Conventional methods to monitor frequency of optical signals employ two optical filters with interleaved transmission characteristics and intensities of optical signals passed the optical filters are compared.
The conventional methods can be used with single channel systems but cannot be applied to multiple channel systems like systems with wavelength division multiplexing technique.
Conventional frequency monitoring methods for multiple channel systems include a method using channel crossover characteristic of AWG (arrayed-waveguide grating) and a method using AOTF (accusto-optic tunable filter).
However, the method using channel selection characteristic of AWG requires expensive ANG and the method using AOTF is not able to monitor frequency and optical output at systems with 100 GHz channel since bandwidth of AOTF is bigger than 1 nm.
An apparatus monitoring frequency of WDM signals using pilot tone and two interleaved solid etalon filters for efficient management and maintenance of wavelength division multiplexing optical transmission network is provided.
Desirably, the apparatus for optical frequency monitoring includes splitting means, a plurality of optical filtering means, optical detecting means, and frequency detecting means. The splitting means splits optical signal and the optical signal includes a pilot tone. The plurality of optical filtering means filters an output of the splitting means with particular passbands. The optical detecting means converts an output of the optical filtering means into an electrical signal. The frequency detecting means detects an intensity of the pilot tone from an output of the optical detecting means and calculates optical frequency.
Desirably, the frequency detecting means includes analog-digital converting means, fast Fourier transforming means, and frequency calculating means. The analog-digital converting means converts the output of the optical detecting means into a digital signal. The fast Fourier transforming means performs fast Fourier transform on the digital signal. The frequency calculating means measures the intensity of the pilot tone from an output of the fast Fourier transforming means and calculates frequency from the intensities of the pilot tone.
Desirably, the frequency detecting means includes electrical filtering means, power detecting means, and frequency calculating means. The electrical filtering means passes the corresponding pilot tone of optical signal from the output of the optical detecting means. The power detecting means converts intensity of the pilot tone from an output of the electrical filtering means into the voltage. The frequency calculating means calculates frequency from the voltage from the power detecting means.
Desirably, the frequency calculating means calculates frequency by using the ratio of the intensities of the pilot tone.
Desirably, frequency calculating calculates frequency by using difference of the intensities of the pilot tone.
Desirably, the optical filtering means is a plurality of filters having interleaved transmission characteristics.
Desirably, the optical filtering means is a solid Fabry-Perot etalon filter and transmission characteristics of the Fabry-Perot etalon filter have periodical characteristics in response to wavelength of optical signal.
Desirably, the optical filtering means is an optical fiber Fabry-Perot etalon filter and transmission characteristics of the optical fiber Fabry-Perot etalon filter have periodical characteristics in response to wavelength of optical signal.
Desirably, the optical filtering means is an arrayed-waveguide grating and resonant frequency of the arrayed-waveguide grating is similar to channel of wavelength division multiplexed signal.
Desirably, the optical filtering means is an optical filter and the optical filter operates with frequency range of wavelength division multiplexed optical signal.